1. Field of the Invention
The present invention relates to a method for manufacturing a gas turbine blade having at the inside thereof a cooling pass, in which Al coating is applied onto the partial area of an inner surface of the cooling pass; and a gas turbine blade in which Al coating is applied onto a partial area of the inner surface of a cooling pass by this method.
2. Description of the Related Art
In recent gas turbines, a tendency of heightening the temperature of combustion gas therein has been advancing to aim for a rise in the efficiency of the turbines. The combustion gas temperature has already exceeded the melting point of a heat resistant alloy used in their turbine blade, turbine nozzle, and other components. Thus, a control is made in such a manner that a cooling pass is located inside the gas turbine blade or gas turbine nozzle and air is circulated in the pass to cool the blade or nozzle, thereby keeping the blade or nozzle at the allowable temperature of the heat resistant alloy, or lower.
However, even when gas turbines are cooled, a rise in the temperature of the inner surface of their cooling pass is unavoidable with the advance of the tendency of heightening the temperature of the gas turbines. Thus, in the present circumstances, the gas turbine temperature is close to the allowable temperature of the heat resistant alloy. In particular, the inner surface of the cooling pass, which is exposed to high temperature, is deteriorated by oxidation, so that the following two problems are caused:
The first problem of the two is the wastage of the pass by the oxidation. Specifically, bulges and depressions are made in one selected from various patterns on the inner surface of the cooling pass, and a convection caused by the bulges and depressions enhances the cooling effect. However, the inner surface of the cooling pass is oxidized so that the surface of the bulges and depressions undergo wastage. When the wastage causes a change in the shape of the inner surface, the gas turbine blade is directly lowered in cooling efficiency.
The second problem is the blockage of the cooling pass by oxides peeled from the inner surface. Specifically, the cooling pass is generally narrow and meandering to be complicatedly configured, so that oxides peeled therefrom deposit easily onto a cooling-air flow-rate change zone of the cooling pass. The depositing oxides block the pass to hinder the cooling.
Against the problems caused by the oxidization, a countermeasure is generally used in which for the formation of a protective oxide made of Al2O onto the inner surface of the cooling pass, the inner surface of the cooling pass is coated with Al or an alloy thereof, thereby improving this surface in oxidation resistance.
As described above, the cooling pass is complicatedly configured inside the blade, and is further narrow; thus, it is difficult to apply, to the pass, thermal spraying, electron beam physical vapor deposition (EB-PVD), overlay coating based on electroplating, or any other method that is generally used to give oxidation resistance to the outer surface of a blade.
Thus, it is general to use, as a method for applying Al coating onto the inner surface of the cooling pass, chemical vapor deposition (CVD) of supplying Al in the form of gaseous halogen to precipitate Al onto the inner surface of the cooling pass.
A main purpose of the Al coating is to improve the inner oxidation resistance of the inner surface of the cooling pass by the protective oxide made of Al2O3. However, it depends on cooling-temperature conditions whether Al2O3 is produced as an external oxide scale that fulfils a function as a protective layer that attains thermodynamic stability which Al2O3 has, or Al2O3 is produced as an internal oxide inside the coating layer not to fulfil a function as a protective layer.
Al2O3 is generally thermodynamically stable at about 900° C., and is further produced as an external oxide scale that fulfils a function as a protective layer. However, at temperatures of about 700° C., Al2O3 is thermodynamically unstable. In other words, Al2O3 itself is not easily produced, and even when Al2O3 is produced, produced Al2O3 remains as an internal oxide inside the coating layer not to fulfill a function as a protective layer.
When Al2O3 is produced as the internal oxide, the oxidation of Ni, Cr, and any other alloying-element that is contained in the substrate of the blade unfavorably advances, so that advantageous effects based on the Al coating are not produced. On the contrary, the oxidization may be accelerated.
In a gas turbine blade, centrifugal force, thermal stress based in a difference in temperature between combustion gas and cooling air, and other stresses are generated. These stresses are complicatedly changed by starting or stopping the action of the gas turbine. Thus, the fatigue strength of the blade is very important. In connection with the fatigue strength, a high-Al-concentration alloy layer formed by Al coating, such as NiAl, is poor in toughness and ductility to decline the blade in strength reliability, in particular, fatigue strength.
The degree of this decline in the fatigue strength depends on the material of the blade. However, the degree is remarkably increased at low temperatures of about 600° C. or lower.
Apart from the above, the temperature distribution of the inner surface of the cooling pass of the gas turbine blade is uneven and depends on the design of the blade. However, almost all of temperatures of the blade are designed to be about 700° C. Edges of the blade, or the leading edge and the trailing edge of the blade, and other high-temperature regions thereof are designed to be adjusted to about 900° C.
Accordingly, when Al coating is applied onto a low-temperature portion of the inside of the cooling pass, the region is improved in oxidation resistance. However, the degree of the improvement is small. About the fatigue strength, the application of the Al coating makes the strength low to decline the reliability of the gas turbine blade.
However, in conventional Al coating techniques using CVD, Al is supplied through a gas fluid containing Al, and the Al-supply aims for the application of coating onto the whole of the inner surface of a cooling pass. Thus, Al coating is unfavorably applied also to a region, inside the cooling pass inner surface, turned into a temperature range where a bad effect is produced on the reliability of the blade by the Al coating.
In order to avoid such an evil based on Al coating, it is necessary to apply Al coating partially only into a portion of the inner surface of the cooling pass where an advantageous effects based on the Al coating can be expected.
In the case of Al coating onto the outer surface of a gas turbine blade, a partial application thereof can be attained by masking or some other method. However, about the inner surface of a cooling pass of the blade, which is narrow and is complicatedly configured, it is very difficult to apply a proper masking thereto highly precisely. Furthermore, the removal of the applied mask is likewise difficult. When this mask remains inside the cooling pass, the cooling pass is blocked.
Therefore, it is an urgent necessity to develop an Al coating method capable of improving a gas turbine blade in reliability by attaining, in Al coating onto the inner surface of a cooling pass of the gas turbine blade, both of an improvement in the oxidation resistance of the blade and a restraint of a decline in the fatigue strength thereof.
Japanese Unexamined Patent Application Publication No. JP 2006-169631 (Patent Document 1) discloses a technique in which in an internal pass of a turbine blade, an Al coat relatively large in thickness to be tough is formed inside the blade-shaped region of the blade while an Al coat relatively small in thickness is formed inside the root thereof.
However, the Al coat disclosed in the literature is varied in thickness in accordance with each of its portions. Accordingly, in the same manner as described above, the Al coat is formed also in a portion of the blade that does not require any Al coat. As described above, in the inner surface of the cooling pass, Al coating cannot be partially applied only into its portion where advantageous effects based on the Al coating can be expected by the technique disclosed in the literature.
Furthermore, Patent Document 1 does not naturally disclose a technique of setting an Al coating portion in accordance with the temperature distribution of the inner surface of the cooling pass. Accordingly, even when the technique disclosed in Patent Document 1 is used, it is never possible to attain both of an improvement in the oxidation resistance and a restraining of a decline in the fatigue strength.
In light of the above-mentioned problems, the present invention has been made. An object thereof is to provide a method for manufacturing a gas turbine blade which provides a gas turbine blade having a cooling pass which has an inner surface coated partially with Al, capable of improving a gas turbine blade in reliability by attaining both of an improvement in the oxidation resistance of the blade and a restraint of a decline in the fatigue strength thereof.